The present invention relates generally to discrete multi-tone communication systems in which a central unit services a plurality of remote units. More specifically, it relates to methods for coordinating upstream communications from the remote units.
Discrete Multi-Tone (DMT) data transmission schemes have been shown to facilitate high performance data transmission. Among the benefits of DMT architectures is that they have high spectral efficiencies and can adaptively avoid various signal distortion and noise problems. Since they have very high data transmission capabilities, in most applications selection of a DMT data transmission scheme will provide plenty of room for the expansion of service as the demands on the data transmission system increase. Hence, discrete Multi-Tone technology has applications in a variety of data transmission environments. For example, at the time of this writing, the Alliance For Telecommunications Information Solutions (ATIS), which is a group accredited by the ANSI (American National Standard Institute) Standard Group, is nearing finalization of a discrete multi-tone based standard for the transmission of digital data over Asymmetric Digital Subscriber Lines (ADSL). The standard is intended primarily for transmitting video data over ordinary telephone lines, although it may be used in a variety of other applications as well. The pending North American Standard is referred to as the T1E1.4 ATIS Standard, and is presently set forth in Standard Contribution No. 94-007, rev. 8, dated January of 1995, which is incorporated herein in its entirety.
Transmission rates under the ADSL standard are intended to facilitate the transmission of information at rates of at least 6 million bits per second (i.e., 6+Mbit/s) over twisted-pair phone lines. The standardized discrete multi-tone (DMT) system uses 256 "tones" or "sub-channels" that are each 4.3125 kHz wide in the forward (downstream) direction. In the context of a phone system, the downstream direction is generally considered transmissions from the central office (typically owned by the telephone company) to a remote location that may be an end-user (i.e., a residence or business user). In other systems, the number of tones used may be widely varied. However when IFFT modulation is done, typical values for the number of available sub-channels (tones) are integer powers of two, as for example, 128, 256, 512, 1024 or 2048 sub-channels.
The Asymmetric Digital Subscriber Lines standard also contemplates the use of a reverse signal at a data rate in the range of 16 to 800 Kbit/s. The reverse signal corresponds to transmission in an upstream direction, as for example, from the remote location to the central office. Thus, the term Asymmetric Digital Subscriber Line comes from the fact that the data transmission rate is substantially higher in the forward direction than in the reverse direction. This is particularly useful in systems that are intended to transmit video programming or video conferencing information to a remote location over the telephone lines. By way of example, one potential use for the systems allows residential customers to obtain video information such as movies over the telephone lines or cable rather than having to rent video cassettes. Another potential use is in video conferencing.
The discrete multi-tone (DMT) transmission scheme has the potential for use in applications well beyond data transmissions over telephone lines. Indeed, DMT can be used in a variety of other digital subscriber access systems as well. For example, it may be used in cable based subscriber systems (which typically use coaxial cable) and wireless subscriber systems such as digital cellular TV. In cable systems, a single central unit (central modem) is typically used to distribute digital signals to more than one customer, which means more than one remote unit (remote modem). While all of the remote modems can reliably receive the same digital signals, the upstream transmissions must be coordinated to prevent confusion at the central modem as to the source of the upstream signals. In some existing cable systems (which do not use discrete multi-tone transmission schemes), each remote unit is given a dedicated frequency band over which it is to communicate with the central station. However, such an approach is inherently an inefficient use of transmission bandwidth and typically requires the use of analog filters to separate transmissions from the various remote units. Other existing cable systems use a single wide band for all remote units, which use time division multiple access (TDMA) to access the upstream channel. This approach is inefficient because of the lower total capacity of the single channel and because of the time required for the accessing process. Stationary digital cellular transmission systems face similar obstacles. The ability to access the channel on both a time- and frequency-divided basis would more efficiently utilize the transmission channel. However, the inherent multiplexing nature of DMT has previously restricted its application to point-to-point transmission because transmissions from different sources must be synchronized for the all-digital multiplexing to function properly.
ADSL applications have the potential for a similar problem, although it is typically more limited in nature. Specifically, a single line may service a plurality of drop points at a particular billing address (which may typically be a home or an office). That is, there may be several telephone "jacks" through which the user may wish to receive signals. To facilitate service to multiple locations (jacks) over a single line, the use of a master modem has been proposed to facilitate synchronization. However, this is perceived as being a relatively expensive and undesirable solution. Accordingly, it would be desirable to provide a mechanism in discrete multi-tone data transmission systems that facilitates the synchronization of signals from a plurality of remotes so that a central unit can coordinate and reliably interpret signals sent from the remotes.
One method of synchronization remote units utilizes the concept of a dedicated overhead bus. That is, one or more dedicated overhead sub-channels are used to facilitate initializing new remote units. This system is described in John M. Cioffi's co-pending U. S. patent application Ser. No. 08/252,829, which is assigned to the assignee of the present application and is incorporated herein by reference. Although the use of an overhead bus works well in some applications, other methods of coordinating multi-point to point transmission are desirable as well.
Another feature of transmission systems currently utilized for communications from a remote unit to a central unit is that they either transmit data at a designated maximum rate (frequency-division multiplexing), or they transmit data in packets of a particular size (time-based multiplexing). They do not permit both. This limits the efficiency of the use of the transmission channels. Accordingly, it would be desirable to provide a mechanism through which when necessary, a remote unit can specify a desire to transmit at a particular data rate and when the data rate is not a concern, the remote unit may indicate that it desires to transmit a designated amount of information.